Corrugated cardboard has a variety of applications as packing material for many products. Corrugated cardboard is multi-walled. The most common corrugated board has three paper board walls. The two outside walls are usually called liners and the inner corrugated wall is commonly called the corrugated or fluted medium or just fluting.
One of the problems currently found in the industry, particularly in the trade involving products where the packing boxes made of corrugated cardboard have to withstand humid conditions, is that upon contact with water, the cardboard tends to absorb sufficient water to lose or significantly reduce its rigidity, whereby the boxes become ineffective for this intended use due to deformation and even eventual destruction.
In order to provide the corrugated cardboard with a water-proof quality, the liners, and sometimes the fluting, are currently coated with a variety of materials, mainly waxes and paraffins, thereby seeking relatively inexpensively to form a film of impermeable material (which retards the water from coming into contact with the underlying cardboard). A significant disadvantage in the standard use of paraffins arises from many problems encountered when used cardboard is recycled. Paraffin tends to form masses in the repulping vat which cannot be easily handled and must be separated.
It is also known that in order to provide stacking strength to packing boxes made of corrugated board, the liners and fluting are made of cardboard having a predetermined weight basis to provide the necessary strength, and additionally the flutes of the corrugated medium board in the box side walls are oriented in a vertical direction. When utilizing higher weight basis for the component materials, the vertical stacking strength will, within limits, be higher, but the cost also will be greater. It is therefore desirable to enhance the strength of the cardboard without increasing its weight basis.
Documents cited in this text, and all documents cited or referenced in the documents cited in this text, are incorporated herein by reference. Documents incorporated by reference into this text or any teachings therein may be used in the practice of this invention.
U.S. Pat. No. 3,085,026 discloses a wax composition for coating corrugated cartons and sheets thus producing stronger corrugated cardboard containers. The cardboard is impregnated with a composition containing at least 60% by weight of a refined crystalline paraffin wax and from 1 to 15% by weight of a polymer resin selected from the group of petroleum polymer resins and styrene polymer resins; said composition constituting from 26 to 37% of the total weight of the impregnated cardboard.
U.S. Pat. No. 3,260,690 discloses coated paper containers, such as drinking cups, milk cartons and the like for receiving various foods and beverages.
The coating composition comprises (a) a water emulsified styrene-butadiene copolymer alone, or (b) a water emulsified styrene-butadiene copolymer in admixture with water emulsified polystyrene. Polyacrylic resins and polyvinyl acetate may be added to improve the coating's resistance to the presence of oils.
U.S. Pat. No. 3,308,006 teaches a coating for a corrugated medium (fluting) comprising a wax-copolymer blend of a wax containing from about 25 to about 60 wt % ethylene-ethyl acrylate copolymer, ethylene vinyl acetate, butyl rubber, polyisobutilene, butadiene-styrene, and others, wherein the wax is selected from the group consisting of paraffin wax, microcrystalline wax and mixtures thereof.
U.S. Pat. No. 4,117,199 discloses a coated paper having a water-proof coating. The coating composition is an aqueous emulsion containing a synthetic rubber latex and a wax emulsion in an amount of the solid wax of 5 to 200 parts by weight per 100 parts by weight of the solid synthetic rubber.
U.S. Pat. No. 5,332,458 discloses a process for increasing the strength of corrugated cardboard applying a strength enhancing isocyanate resin to the fluted medium. This patent describes the general process of manufacturing a corrugated cardboard with an enhanced resistance.
U.S. Pat. No. 5,658,971 describes an aqueous coating composition comprising up to about 85% by weight of a latex of a styrene-butadiene copolymer, or a latex of a carboxylated styrene-butadiene copolymer, and a wax. The composition may also comprise sodium alginate and polyethylene.
U.S. Pat. No. 5,750,237 describes a double-faced liner board that is waterproof and which can be easily recyclable in contrast to cardboard waterproofed by wax. The top layer of the liner board is coated by a coating comprising an emulsion of a curable butadiene-styrene copolymer and TiO2, the fluted medium layer is coated with an aqueous composition containing an emulsion of wax particles and a binder, preferably sodium alginate; and the bottom layer has a coating formed from an aqueous composition comprising about 10 to 85 wt % of a liquid curable styrene-butadiene copolymer and a wax.
U.S. Pat. No. 5,869,192 teaches an aqueous curable composition comprising a pre-polymer of a styrene-butadiene copolymer and TiO2, forming a waterproof coating.
U.S. Pat. Nos. 5,635,279; 5,626,945 and 6,066,379 describe a water-repellant coating which includes a mixture of a polymer matrix and a pigment. The wax component may be paraffin or polyethylene wax. The polymer matrix may be polyethylene-butadiene polymer. The pigment is selected from the group consisting of aluminum trihydrate, barium sulfate, calcium carbonate, mica (potassium aluminum silicates), nepheline syenite (sodium potassium aluminosilicate), finely ground silica sand and other natural and synthetic types of silicates including talc (magnesium silicates), wollastonite (calcium metasilicates), bentonite (montmorillonite, smectite) and clay. A cardboard is described having a water repellant coating which includes a polymer matrix/wax mixture applied as an aqueous formulation. The coating composition consists essentially of a wax component selected from paraffin wax or polyethylene wax emulsions; a polymer matrix of polymer chains comprising a polystyrene-butadiene polymer copolymerized with a monomer having pendant carboxylic acid groups, which are cross-linked by a zinc ion.
U.S. Pat. No. 6,429,240 discloses a water-borne resin composition which can be readily applied to paper substrates and which provides enhanced strength and rigidity for the paper substrate. The coated paper is repulpable and recyclable. The coating composition comprising a water-borne resin selected from the group consisting of coal, tar, petroleum, turpentine feed stocks, and mixtures thereof; and a natural resin selected from the group consisting of resins, fossil resins, mined resins, secretion products from insects, and chemical derivatives and adducts of such natural resins.
U.S. Pat. Nos. 6,541,556 and 6,143,113 disclose a wax-free coating composition for corrugated cardboard comprising a styrene-acrylate copolymer and a C14-C18 fatty acid complex of a metal ion such as chromium.
U.S. Pat. No. 6,722,560 teaches a box of corrugated cardboard having a single moisture vapor barrier comprising a composition of polymers and ground-up mica.
U.S. Pat. No. 6,794,016 discloses a water-resistant corrugated fiberboard capable of being recycled after its use. A water-resistant layer is formed on at least one of the surfaces of the corrugated fiberboard by applying a coating containing a synthetic resin emulsion and a pigment. In a preferred embodiment of the invention of this publication, the synthetic resin emulsions are styrene/butadiene copolymer, styrene/acrylic copolymer and ethylene/vinyl acetate copolymer. A pigment may be added from the group consisting of precipitated calcium carbonate, calcium carbonate, heavy calcium carbonate, silica, clay, kaolin, talc, mica, sericite and aluminum hydroxide.
From all these, it can be seen that there is a continuing long-felt need in the corrugated box industry to find a truly low-cost and effective coating composition which adds strength to the cardboard, while being repulpable, and therefore contributing to an ecological recycling of used cardboard packings.
The invention also has application as a paper strengthener whereby for a given stacking resistance of corrugated packing boxes, the coating of the present invention allows for manufacturing said boxes utilizing a thinner paper than otherwise would be required to have such compression resistance. This provides an important economic advantage to the manufacturer of the packing boxes.
It is thus desirable to increase the strength of the corrugated board using the minimum weight basis of cardboard by enhancing the strength of at least one layer (i.e. ply) of the corrugated board.